1. Field of the Invention
The present invention is generally drawn to the repair of boiler elements by welding and more particularly to tube section replacement using arc welding.
2. Description of the Prior Art
The conditions of energy production have changed over the years since the first nuclear reactors were put into service (especially in the United States). Many nuclear power plants are approaching the end of their design life and the licensed operators of these plants are seeking to extend the life of the plants (and plant components). Many of these components were not designed with concerns of replacement and repair, and some components have been subjected to unforeseen necessity for repair due to material degradation. As the demand for these repairs has risen, new technologies to address these repairs have developed.
For this technology to become reality, an all encompassing approach of low profile welding machine design and process development needs to be addressed to include welding various material types and dissimilar metal welding joints, so that the end product is capable of performing in a wide variety of applications. Enabling technologies for this process include:
Development of robust welding procedures with increased penetration capabilities;
Ability to weld various material types;
Elimination of wire feeding mechanism for compact tool design.
The application for this technology will be varied within the power generation industry. Any application that has tight tolerances on the radial clearance between tubes would be applicable. One example of this would be the boilers of fossil fuel plants, where tubing with limited tube to tube radial clearance require maintenance. Another application would the feeder tubes on CANDU reactors.
CANDU reactors are plagued by an unforeseen problem that has arisen in the feeder tubes of the reactor. Flow assisted corrosion (FAC) on the feeder tubes is forcing wholesale replacement or remediation/mitigation strategies to be developed for the feeder tubes. Feeder tube design imposes stringent welding requirements (i.e.0.000″-0.030″ maximum root penetration) on the repair/replacement approaches. In addition to stringent penetration requirements, the feeder tube location and geometry presents challenges for tooling design. CANDU feeder tubes have an extremely compact arrangement with an outside diameter of inches. FIG. 1 and FIG. 2 illustrate the layout of feeder tubes within a CANDU reactor.
Known technologies for such applications include the high energy density welding processes that offer the ability to achieve high penetration capabilities through a keyhole welding process. The keyhole process is generally not preferable for orbital applications because parameter development (i.e. ramp up and ramp down) is difficult for proper closure of the keyhole at the end of the welding process. These processes also require high capital equipment expenditures and additional process training due to the fact that the majority of the welding workforce is not familiar with these processes.
Gas Tungsten Arc Welding (GTAW) is an arc welding process that utilizes a non-consumable Tungsten electrode along with an inert shielding gas to provide a molten weld pool that produces a weld deposit of high quality. High quality weld deposits and relatively low cost equipment have made GTAW an indispensable tool in many industries.
As with any process, GTAW has limitations including low deposition rates (slow production rates) and limited penetration capabilities. At material thicknesses greater than 3 mm, penetration capabilities of the process become questionable. Given a tube-to-tube welding operation with a material thickness greater than 3 mm, the weld prep would change to require a multipass welding procedure. Typically the weld joint chosen for an application that can only be welded from a single side (i.e. the tube OD) would be a single-V groove. The single-V groove geometry would be dictated by the number of welding passes required to fill the joint with an appropriate size weld deposit and the design of the equipment for filing that groove. Increased penetration would create efficiency in both process time and set-up time. As the penetration capabilities increase, the requirements for multipass welding decrease. For example, if the GTAW process can penetrate 3/16″ material and provide sufficient backing during the welding process, the total arc time could possibly be cut by ⅓ due to the fact that general welding procedures would require a 3-pass weld procedure at this material thickness. Real time savings would not be realized by the welding time alone. With increased penetration, the weld prep can be changed from the single-V prep, which requires a machining operation, to a square groove weld.
GTAW can be utilized with or without filler material, however, welding on steels that have not been fully deoxidized (i.e. rimmed steels) can cause porosity problems, which can be mitigated utilizing filler materials that contain deoxidizers. Filler metals also provide additional material that is needed for weld reinforcement, both ID and OD, when required by the design specifications.
GTAW filler materials are specified in such a manner so that its composition contains elements that assist during the welding process by imparting specific mechanical properties to the weld deposit or by assuming that the weld deposit remains clean. This is especially beneficial when welding materials that have been processed in a manner that is not friendly to welding (i.e. dirty steels). Generally, filler materials are added to the GTAW process in one of two states, filler wire or consumable insert.
Filler wire is deposited into the GTA weld pool via a spool of wire that is fed into the arc from a wire spool through a length of conduit and a wire feeding mechanism. For this process to work correctly, the wire has to be placed in the vicinity of the weld pool near the arc to properly melt the wire in the molten weld pool. Such techniques make for a large weld apparatus which does not readily fit narrow spaces such as CANDU reactor feeder tubes.
Consumable inserts act as pre-placed filler material. Consumable inserts are offered in a wide variety of shapes, sizes and material compositions. Known suppliers of such inserts include Arcos, ITT Grinnel, Weldring, and Robvon Inserts. FIG. 5 shows five various shapes of consumable inserts that are currently manufactured in ribbon form including a rectangle, a T-shape, a Y shape, a rounded pin shape and a ½ pin shape. These inserts are intended for filler in V or square groove welding applications and would not be usefull in abutting tube joining as is done in the CANDU filler tube replacement.
The GTAW process is limited to melting the consumable insert in much the same way it is limited to melting the base material. The consumable insert size will be limited to the penetration capability of the process. If the penetration capability of the process can be increased, then the size of the consumable insert can be increased. The consumable insert is designed to mate to a corresponding material thickness. Increased penetration capability increases the design envelope for the consumable insert, which increases the compatibility of material thicknesses for that type of consumable insert.
Penetration fluxes are used to increase weld penetration. These fluxes are oxides applied to the surface of the material to be welded that provide an increased ability for the process to penetrate the material. The primary benefits of using flux derive from its ability to increase autogenous GTA weld penetration-to-width ratio by a factor of 2 to 3. This significantly reduces welding times and simplifies weld joint preparation, making it possible to use a square butt joint where a groove prep was previously required. Weld distortion is also reduced due to a more symmetrical weld cross section. Flux is relatively inexpensive and easy to use; therefore, the costs for implementation are nominal. FIG. 4 exhibits the effects that the GTAW penetration flux can have on the autogenous GTAW process. Note that the weld cross section transforms from a partial penetration weld, located on the left-hand side of the pipe shown in the photograph, to a fully penetrated weld on the right-hand side of the pipe shown in the photograph. However, todate fluxes have not been directly applied to consumable inserts prior to welding but were added by a wire feeder of the flux. Thus to provide an efficient fast weld in a space restricted area a process using GTAW in combination with a suitable consumable insert and the use of flux was needed to provide the penetration capabilities required by the design of the joint and the appropriate material properties (imparted by the material composition of the filler material) for a fully functional welding design.